This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in Japanese Patent Application No. 2002-077860 filed on Mar. 20, 2000.
1. Field of the Invention
The present invention relates to brushless motors for driving storage disks, such as CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-RAM, and DVD-RW. More particularly, the present invention relates to brushless motors in which magnetic field variations, generated by a rotor magnet, are detected by a rotational position detecting means that is disposed in a particular position on the stator side, such that the stator current can be switched using the detected result.
2. Background Art
As shown in FIG. 6, by way of example, a brushless motor used to drive a disk comprises a stationary frame 100, a shaft 104 rotatably supported by the stationary frame 100 through a bearing 102, a rotor 106 mounted to the shaft 104 and rotating in union with the shaft 104, a rotor magnet 108 attached to the rotor 106, and a stator 110 supported by the stationary frame 100 in a position facing the rotor magnet 108.
The stator 110 comprises a stator core 112 and stator windings 114 having, for example, 3 phases, which are wound over the stator core 112. Also, the rotor magnet 108 is magnetized with multipolar magnetization, in which the rotor magnet is magnetized into different magnetic poles alternating in the circumferential direction. The direction of current supplied to the stator winding 114 of each phase is changed in correspondence with the rotation of the rotor magnet 108. The torque of the motor is obtained through repeated attraction and repulsion between the magnetic poles of the stator 110 and the magnetic poles of the rotor magnet 108.
In order to make the brushless motor rotate, the current supplied to the stator winding 114 of each phase must be switched in sync with the rotation of the rotor magnet 108. The timing for switching the current is generated by detecting variations of a magnetic field generated by the rotation of the rotor magnet 108. The field detecting means 116 is disposed in a particular position on the stationary side. A Hall device 116 is used as one known example of field detecting means for that purpose.
The Hall device 116 generates a voltage depending on the amount of magnetic flux penetrating the Hall device. Accordingly, the greater a change in the magnitude of the terminal voltage, more precisely a change in the amount of magnetic flux penetrating the Hall device can be measured with higher sensitivity. In the brushless motor, variations of magnetic flux caused by the rotation of the rotor magnet 108 are detected by the Hall device 116 that is disposed in a particular position on the stationary side. Detection sensitivity in the rotor rotation can be increased by arranging the Hall device 116 at a position where the magnetic flux penetrating the Hall device is maximally changed with the rotor rotation.
Further, the rotor magnet 108 is multipolar-magnetized such that different magnetic poles are alternating in the circumferential direction and in the radial direction. In a cross-section of the rotor magnet 108 shown in FIG. 6, the radially inner side of the rotor magnet is magnetized into an N (or S) pole and the radially outer side thereof is magnetized into an S (or N) pole. Then, lines of magnetic force generated by the rotor magnet 108 are radially extended from both poles and are deflected, to a large extent, depending on the arrangement of magnetic bodies disposed in the surroundings of the rotor magnet. It is usually thought that the sensitivity in detecting the rotor rotation is increased by arranging the Hall device in a position directly below, and closer to, the rotor magnet. In positions away from the position directly below the rotor magnet, the rotation detection sensitivity is reduced, while it is relatively increased by arranging the Hall device in a position where the lines of magnetic force are concentrated (and hence the density of magnetic flux is relatively high).
Recently, notebook personal computers capable of handling CD-ROMs or the like have been commercialized. The size and thickness of these disk drives for driving CD-ROMs or the likes have been reduced. Correspondingly, there is a demand for a reduction in the size and thickness of the brushless motors that are to be incorporated in these disk drives.
However, because the Hall device 116 is disposed directly below the rotor magnet 108, as shown in FIG. 6, the presence of the Hall device 116 impedes an attempt at reducing the motor thickness in the axial direction.
On the other hand, when attempting to move the position of the Hall device 116 radially inward of the rotor magnet 108 to avoid such a drawback, there is not sufficient space to accommodate the Hall device, because the stator windings 114 are disposed radially inward of the rotor magnet as shown in FIG. 7. Also, even if there is sufficient space, it would be difficult to precisely detect the rotor rotation because, in a position away from the rotor magnet, the density of magnetic flux is reduced and, therefore, the sensitivity in detecting the rotor rotation is reduced.
It is an object of the present invention to provide a brushless motor, in which a rotor position detecting device is able to detect changes in the density of magnetic flux that are caused by rotation of a rotor magnet, while reducing the size and thickness of the motor.
According to the present invention, a brushless motor includes a rotor position detecting device disposed between stator teeth, rather than directly below a rotor magnet, so that the distance between a lower end of a rotor magnet and a stationary frame is minimized. This results in a thinner brushless motor than conventional designs. In the preferred embodiment of the present invention, the rotor position detecting device is a Hall device.
Further, according to the present invention, the windings are wound over a stator in larger number on the inner peripheral side than they are on the outer peripheral side thereof. However, the total number of stator windings remains substantially equal to that in a conventional motor, so that sufficient space to accommodate the Hall device is defined between the adjacent teeth of a stator core.
In addition, the Hall device that is disposed in such a space is fixed in a position where the lines of magnetic force generated from the rotor magnet are concentrated (and hence the density of magnetic flux is relatively high), and the magnetically sensitive surface of the Hall device is inclined with respect to the axial direction of a shaft of the motor. This arrangement enables the Hall device to receive the most possible magnetic flux generated during the rotation of the rotor magnet. As a result, the Hall device can detect, with satisfactory accuracy, the timing of switching in a stator current supplied to the brushless motor.
With the arrangements described above, the present invention has succeeded in reducing the size and thickness of the brushless motor.